Use low power WFE when idle.
Set PORT.C to built `-O0` always because it seems to occasinally end
up with interrupts disabled in task code, causing the SYSTICK never to
fire and killing task switching.
No need for dynamic exceptions. We don't move the execbase.
Using all the work from @hfellner and others, add FreeRTOS
SMP support.
Allow idling cores through the FreeRTOS FIFO queue to
allow for file system and EEPROM support.
Make delay a weak function so FreeRTOS can override.
Add cycle count support under FreeRTOS using a PIO SM.
Use a task-based approach for handling the USB periodic work
instead of the IRQ-based one in the main core.
Set 8 prio levels so it fits in 3 bits nicely (0..7).
Earle F. Philhower, III